The use of smectic A liquid crystals in display means using a mixed thermal and electrical effect is now well known. Among the materials which can be used for this purpose are diphenyl derivatives such as 4-cyano-4'-octyl diphenyl, which has a smectic A phase and a nematic phase. Other patents of the present Applicant relate to smectic A materials with or without a nematic phase during smectic-isotropic phase transitions. The present invention relates to dopants for incorporating into these materials. In particular, these dopants lead to an increase in contrast, a reduction of the control power and also a reduction of the video voltage.